Heteroepitaxial semiconductor/group-II fluoride/Si structures are potentially useful in a number of semiconductor-on-insulator technologies. As one of the most promising candidates in this area, the CaF.sub.2 /Si couple has attracted considerable attention (see T. Asano and H. Ishiwara, "Epitaxial Growth of Ge Films onto CaF.sub.2 /Si Structures", Jpn. J. Appl. Phys., 21, p. L630, 1982, and R.W. Fathauer, et al., "Heteroepitaxy of semiconductor-on-insulator structures: Si and Ge on CaF.sub.2 /Si( 111)", J. Appl. Phys. 60(11), p.3886, 1986). From a materials preparation point of view, CaF.sub.2 /Si is a relevant choice among the fluorides for deposition on silicon because it fits several important criteria for a good epitaxial system, i.e. small lattice mismatch and similar cubic structure. Previous studies have shown that the chemical and structural properties of the CaF.sub.2 /Si interface vary under different deposition conditions and that these variations can significantly change the electrical characteristics of a system incorporating this interface. Co-assigned patent application 07/704,535 describes a method that allows the growth of films in which the orientation of the CaF.sub.2 /Si film is essentially identical to that of the silicon substrate, a characteristic which is important for good interface properties. Additionally, co-assigned U.S. Pat. No. 5,229,333 discloses a low-temperature growth technique for producing improved CaF.sub.2 /Si interfaces.
A primary concern in obtaining good crystal growth by molecular-beam-epitaxy (MBE) or other related vapor-phase techniques is the growth mode of the film. Both lattice strain and surface free energy help determine whether a film undergoes layer-by-layer growth (Frank-Van der Merwe), islanding (Volmero-Weber), or layer-by-layer growth followed by islanding (Stranski-Krastanov). Deliberate introduction of a surfactant that alters the surface free energy can change the growth mode of a film. Surfactants such as Sb, Ga, As, and Te have been widely studied for improving the epitaxial growth of Ge on Si (See, for example, M. Copel, et al. "Surfactants in Epitaxial Growth", Phys. Rev. Lett. 63(6), p.632, Aug. 1989, or R. Cao, et al. "Microscopic study of the surfactant-assisted Si, Ge epitaxial growth", Appl. Phys. Lett. 61(19), p. 2347, Nov. 1992).